The present invention relates to coating compositions having barrier properties which are useful in packaging applications. The coating compositions of this invention are formed by the reaction of bis-aminosilanes with phenolic compounds, and are particularly useful to reduce the diffusion of gases through organic polymer based packaging materials.
Organic polymers, such as polypropylene and polyethylene terephthalate, have gained wide acceptance in the packaging industry because of their inherent advantages over conventional materials such as glass. However, a need exists to improve the barrier properties of organic polymer films for various packaging applications. More particularly, improvements are sought to prevent the loss of gaseous, oil, and flavor components of compositions packaged with organic polymer film substrates.
Coating compositions containing silane compounds are known to improve the gas, oil, and flavor barrier performance of organic polymer film substrates, for example as described in PCT/BE98/00006, the corresponding US equivalent of which is U.S. Ser. No. 09/341253. Moreover, the adhesion of the coating to the film surface, as well as the improved barrier characteristics provided by the silane coating, are greatly enhanced by exposing the coated film to electron beam radiation.
Useful barrier compositions are described in U.S. Pat. No. 5,215,822, which teaches a methanol solution of a vinyl benzyl amine silane, itaconic acid, and water; coating this solution on a corona treated low density polyethylene film, drying, and then subjecting the coated film to electron beam radiation to graft the coating to the film surface and further improve the barrier properties of the silane coating. However, while this coating gives excellent gas barrier properties at low to moderate relative humidity values, the gas permeability increases drastically at very high relative humidity values.
U.S. Pat. No. 5,434,007 teaches a silane resin coated on a plastic film, where the silane resin is composed of a monofunctional acrylate and an amino functional silane.
U.S. Pat. Nos. 5,260,350 and 5,374,483 relate to a silicone coating composition which, when cured on a solid substrate either by ultraviolet or electron beam radiation, provides a transparent abrasion resistant coating firmly adhered thereon. The silicone coating is prepared by reacting at least one multifunctional acrylate monomer with an amino-organofunctionalsilane, mixing the modified silane with at least one acrylic monomer and thereafter adding colloidal silica.
JP (Kokai) publication 7-18221 published on Jan. 20, 1995 teaches a surface treatment composition for gas barrier comprising an amino functional silane and a compound having an aromatic ring or hydrogenated ring.
These coatings represent a significant advance in the art. However, it has been observed that while the barrier properties of the prior art coatings are excellent in environments at relative humidities of 80% or less, their performance suffers significantly at relative humidities of 90% or more.
The present inventor has surprisingly discovered that the reaction products of an amino functional silane and a phenolic compound give excellent gas barrier properties at low to moderate relative humidity values, as well as excellent gas barrier properties at very high relative humidity values of 90% or more.
Coating compositions for improving barrier properties of organic polymer films based primarily on the reaction product of amino functional silanes and phenolic compounds heretofore are not known. Amino functional silanes are commonly used as surface treatments of silicate based materials (such as glass or silica) to enhance the adhesion of a wide variety of organic polymers. Examples of the type of organic polymers reacted with amino functional silane treated silicate materials includes phenol-formaldehyde polymers. Furthermore, the addition of phenolic compounds to phenol-formaldehyde-resin coating compositions are known. In particular, U.S. Pat. No. 4,062,690 teaches a coating composition for glass fibers based on phenol-formaldehyde-resins containing at least one monocyclic or polycyclic aromatic compound having at least three hydroxyl groups on the aromatic ring. While the ""690 patent further teaches the treatment of the glass fibers with an amino functional silane, it does not specifically describe or suggest the reaction product of a amino functional silane with a non-resin phenolic compound is useful to improve the barrier properties of organic polymer films. Rather, the ""690 teaches the necessity of mixing a phenolic compound in a phenol-formaldehyde-resin to obtain a coating composition.
Silamines have been reacted with phenols to create curing agents for epoxide resins, as taught in U.S. Pat. No. 4,393,180. However, these silamines differ from the amino functional silanes of the present invention in that they do not contain an alkoxy group and have not been suggested for improving the barrier properties of organic polymer films.
The present invention is directed to a composition, useful for improving the barrier properties of organic polymer films, prepared by reacting;
(A) a bis-aminosilane and
(B) a phenolic compound
to form a reaction product, wherein the bis-aminosilane has at least one molecule of the formula;
R1bX3-bSixe2x80x94Zxe2x80x94SiX3-bR1b
wherein Z is R2NH(R2NH)pR2, each R1 is a hydrocarbon group, each X is an alkoxy group with 1 to 4 carbon atoms, an oxime group or an acyloxy group, each R2 is a divalent hydrocarbon group having 1 to 12 carbon atoms; b is from 0 to 3 and p is 0 or 1.
The composition can be applied to a variety of substrates used in packaging applications. The composition can be cured by further heating in the presence of moisture.
The present invention also teaches a method for preparing substrates with improved barrier properties by coating a variety of substrates used in packaging applications with the inventive compositions.
The substrates prepared by the method of the present invention show improved resistance of the substrate to transmission of gases and aromas there through. For example, a 30 micrometers uncoated biaxially oriented, corona treated polypropylene film is generally found to have a permeability to oxygen of 1200 cc/m2/day as measured according to ASTM D3985-81 at 90% relative humidity. With the preferred embodiments of the present invention, the oxygen transmission rate of the same film is reduced to less than 1 cc/m2/day as measured at 90% relative humidity. As used herein, the terminology xe2x80x9cimproved barrierxe2x80x9d refers to a coating which can reduce oxygen transmission rate of the aforementioned un-coated polypropylene film from 1200 cc/m2/day to less than 100 cc/m2/day as measured at ASTM D3985-81 measured at 90% relative humidity.
The bis-aminosilane useful as component A) in the composition of the present invention are described by the formula:
R1bX3-bSixe2x80x94Zxe2x80x94SiX3-bR1b
wherein Z is R2NH(R2NH)pR2. In this formula each R1 is preferably a monovalent hydrocarbon group having 1 to 10 carbons, for example a saturated or unsaturated aliphatic or aromatic group, for example alkyl, alkenyl, or phenyl groups; Each X is an alkoxy group with 1 to 4 carbon atoms, an oxime group or an acyloxy group. X is preferably an alkoxy group, with methoxy and ethoxy as the preferred alkoxy groups. R2 is a divalent hydrocarbon group having 1 to 12 carbon atoms, preferably each R2 has from 2 to 3 carbons. Each b is from 0 to 3, but is preferably 0, and p is 0 or 1. The best results are obtained by use of compounds in which each X is a methoxy group, each R2 is a propylene group, b is 0, and p is 0, i.e. when the compound is bis-(xcex3-trimethoxysilylpropyl)amine, such as Silquest A1170 supplied by Witco/OSi, (Greenwich, Conn.). In another embodiment, the bis-aminosilane can be bis-[(3-trimethoxysilyl)propyl]-ethylenediamine, such as bis-TMSEDA from Gelest.
The bis-aminosilanes of the present invention may also be referred to as disilylated secondary amines, and can be prepared by processes known in the art, such as U.S. Pat. Nos. 2,832,754, 2,920,095, and 5,101,055.
Component B) of our composition is a phenolic compound. One skilled in the art recognizes phenolic compounds to be any compound having a structure with at least one hydroxy group substituent on an aromatic ring. The inventors believe any phenolic compound will suffice for reaction with the bis-aminosilanes described above to form the compositions of this present invention. While not to be bound by any theory, the inventors believe the hydroxy group of the phenolic compound reacts with the alkoxy group of the bis-aminosilane, liberating alcohol (corresponding to the alkoxy group on the aminosilane) and forming a complex. The complex unexpectedly provides enhanced physical properties that make them useful in the preparation of barrier coatings.
The phenolic compounds of this invention may be further substituted with a variety of chemical groups, such as hydrogen, alkyl, aryl, hydroxy, carboxylic acids, esters, thio, amino, amide, or nitro groups. Preferably, the phenolic compound has two or more hydroxy substituents on its aromatic ring structure.
The phenolic compounds may have one or several aromatic rings in its structure. When the phenolic compound contains polycyclic aromatic rings, the polycyclic aromatic structure is preferably chosen from the group consisting of naphthyl, anthryl, and phenanthryl derivatives. Preferred embodiments of a polycyclic aromatic phenolic compound are 1,5-dihydroxynaphthalene and 2,7-dihydroxynapthalene.
Most preferably, the phenolic compound has one aromatic ring and contains several hydroxy substituents. A specific preferred embodiment is when the phenolic compound is 1,2,3,-trihydroxybenzene, commonly know as pyrogallol.
The components of the present invention can be reacted together in a solvent. The solvent must wet the substrate and should not extend the drying time of the coating beyond what is commercially acceptable. The amount of solvent can range from about 1% to about 99%. Preferably the alcohol is present from about 5 to about 95 parts by weight of the total composition, and most preferably is present from about 70 to about 80 parts by weight of the total composition. In general, alcohols serve as suitable solvents. Preferred solvents are methanol, ethanol, n-propanol, isopropanol, butanol, and 1-methoxy-2-propanol(available as xe2x80x9cDowanol PMxe2x80x9d from the Dow Chemical Co., Midland, Mich.), with methanol as the most preferred.
The coating can be applied in any desired amount, however, it is preferred that the coating be applied in a thickness ranging from 0.05 micrometers to 15 micrometers, the preferred coating thickness range being from about 0.5 to about 7 micrometers. Coating thickness can be determined by Scanning Electron Microscopy or by the use of a profiler (Tencor P-1 Long Scan Profilometer, Tencor Instruments, Santa Clara, Calif.). The coating can be applied to the substrate by any conventional method, such as spray coating, roll coating, slot coating, meniscus coating, immersion coating, and direct, offset, and reverse gravure coating.
The coating can be disposed on a wide variety of substrates, including, but not limited to polyolefins, such as oriented polypropylene (OPP), cast polypropylene, polyethylene and polyethylene copolymers, polystyrene, polyesters, such as polyethylene terephthalate (PET), or polyethylene naphthalate (PEN), polyolefin copolymers, such as ethylene vinyl acetate, ethylene acrylic acid and ethylene vinyl alcohol (EVOH), polyvinylalcohol and copolymers thereof, polyamides, such as nylon, and poly(meta-xylylene adipamide (MXD6) nylon, polyimides, polyacrylonitrile, polyvinylchloride, polyvinyl dichloride, polyvinylidene chloride, and polyacrylates, ionomers, polysaccharides, such as regenerated cellulose, and silicone, such as rubbers or sealants, other natural or synthetic rubbers, glassine or clay coated paper, paper board or craft paper, and metallized polymer films and vapor deposited metal oxide coated polymer films, such as AlOx, SiOx, or TiOx.
The aforesaid substrates are likely to be in the form of a film or sheet, though this is not obligatory. The substrate may be a copolymer, a laminate, a coextruded, a blend, a coating or a combination of any of the substrates listed above according to the compatibility of the materials with each other. In addition, the substrate may be in the form of a rigid container made from materials such as polyethylene, polypropylene, polystyrene, polyamides, PET, EVOH, or laminates containing such materials.
The aforesaid substrates may also be pretreated prior to coating by corona treatment, plasma treatment, acid treatments and flame treatments, all of which are known in the art.
In addition, the compositions of the present invention can be used for a wide variety of packaging containers, such as pouches, tubes, bottles, vials, bag-in-boxes, stand-up pouches, gable top cartons, thermo-formed trays, brick-packs, boxes, cigarette packs and the like.
Of course, the present invention is not limited to just packaging applications, and may be used in any application wherein gas, or aroma barrier properties are desired, such as tires, buoyancy aides, inflatable devices generally, etc.
Any of the foregoing substrates may have a primer or primers applied thereon. The primers are applied to the substrates by methods known in the art such as spray coating, roll coating, slot coating, meniscus coating, immersion coating, and indirect, offset, and reverse gravure coating. Suitable primers include, but are not limited to carbodiimide, polyethylenimine, and silanes, such as N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and aminopropyltriethoxysilane.
While the compositions of the present invention will form films at ambient conditions, optimum results are achieved by heat curing. Generally, the higher the temperature, the faster the coating will solidify. The upper limit to the heating is the temperature at which the substrate will undergo unacceptable distortion. Also, heating will accelerate the rate of hydrolysis of silicon/alkoxy groups and also the rate of condensation of the silicon bonded alkoxy groups with silicon bonded hydroxy groups to form silicon-oxygen-silicon groups. The composition may be dried at room temperature or in an oven at temperatures up to about 140xc2x0 C., with temperatures of from about 60xc2x0 C. to about 120xc2x0 C. being preferred and temperatures of about 60xc2x0 C. to about 80xc2x0 C. being most preferred. Heating time is temperature and solvent dependent and the coating will reach tack free time in 1 to 10 seconds. The heating step serves to evaporate the solvent when used and accelerate the condensation reaction between Sixe2x80x94OH groups and SiOH/SiOR groups.
Various optional additives can be added to the composition to improve various properties. These additives may be added as desired and in any amount as long as they do not reduce the performance of the barrier coatings as illustrated herein. Examples of additives include additional additives as earlier described, antiblock and slip aides such as stearamide, oleamide or polar additives, such as epoxides, polyols, glycidols or polyamines, such as polyethylenimine,and other silanes may be added. Specifically excluded from the scope of the present invention are colloidal silicas and silanes or other molecules having four alkoxy or other hydrolyzable groups disposed on a single silicone or other organometalic atom, such as tetra ethoxy silane, and the like. Wetting agents, such as a polyethoxylatedalkyl phenols may also be added.
The foregoing specification describes only the preferred embodiment and the alternate embodiments of the invention. Other embodiments may be articulated as well. It is expected that others will perceive differences which while differing from the foregoing, do not depart from the spirit and scope of the invention herein described and claimed.